1. Field of the Invention
The present invention relates to a document reader and, more particularly, to a document reader capable of reading images formed on both sides of a document while the document is automatically conveyed.
2. Description of the Related Art
In the conventional art relevant to the present invention, there has been known a document reader for reading images formed on both sides of a document while the document is automatically conveyed, the document reader comprising a document conveyance path, image reading means of an optical reduction system for reading an image formed on a first side of the document, and an image sensor of a contact type for reading an image formed on a second side of the document, wherein the document conveyance path includes, on a part thereof, a flat back-up guide plate urged by urging means, so that the image formed on the first side of the document is read by the image reading means of an optical reduction system and, then, the image formed on the second side of the document is read while the back-up guide plate urges the second side of the document toward the image sensor of the contact type, so as to compensate a disadvantage of the image sensor of the contact type having a small focal depth (see for example, Japanese Unexamined Patent Publication No. 2002-182437).
Examples of an image reading system of a document reader installed in a copying machine or the like generally include a document fixing system and a document moving system.
In the document fixing system, a document is placed on a transparent platen by a user and, then, out of image reading means including a light source, a mirror group, a lens and a CCD disposed under the platen (hereinafter referred to as “image reading means of an optical reduction system”), an image formed on the document is scanned while the light source and the mirror group are moved with respect to the document, so that a light beam reflected on a mirror is focused on the CCD via the lens, whereby the image formed on the document is electronified.
On the other hand, in the document moving system, the document is scanned in a state in which the light source and the mirror group are stationary with respect to the document conveyed by an automatic document feeder (hereinafter abbreviated as an “ADF”) to then pass on the platen; thus, the image is read.
Most of the recent document readers have been provided with the ADF, and further, have been configured in such a manner as to read images formed on both sides of the document to be conveyed by the ADF.
A system for reading images formed on both sides of a document has been exemplified by a system in which an image formed on a first side of the document is read by image reading means of an optical reduction system disposed under a platen while the document passes on the platen and, then, the document is reversed by the use of a switch-back path disposed on the way of a document conveyance path, and an image formed on a second side of the document is read while the document passes on the platen again.
However, in the above-described system in which the document is reversed by the use of the switch-back path, the image formed on the first side is read, the document is reversed by the use of the switch-back path, and the image formed on the second side is read while the document passes on the platen again. Therefore, much time has been required for reading the images formed on both sides.
In addition, the use of the switch-back path complicates a document conveyance control and, further, causes the high possibility of a document jam. The document jam, in particular, has been a problem to be most avoided in the document reader provided with the ADF.
In view of the above-described situations, there has been devised a system in which second image reading means is provided for reading an image formed on a second side of a document on a document conveyance path after an image formed on a first side of the document is read until the document reaches a paper discharge tray such that the images formed on both sides can be read while the document passes on a platen only once, as disclosed in Japanese Unexamined Patent Publication No. 2002-182437.
However, the second image reading means has been disposed on the way of the document conveyance path in the ADF repeatedly opened or closed at frequent intervals with respect to the platen; therefore, there has been a restriction from the viewpoint of an installation space or durability against vibration. As a consequence, there has been usually adopted an image sensor of a contact type superior to the image reading means of the optical reduction system from the viewpoint of the installation space or the durability against the vibration.
In deed, the image sensor of the contact type is superior to the image reading means of the optical reduction system from the viewpoint of the installation space or the durability against the vibration, but is inferior to the image reading means of the optical reduction system from the viewpoint of a resolution or a focal depth.
Consequently, the image formed on the first side read by the image reading means of the optical reduction system is clear; however, the image formed on the second side read by the image sensor of the contact type is liable to become more unclear than the read image formed on the first side.
In view of this, most of the document readers have subjected data on the read images on the first and second sides to image processing, to then output the images, so as to achieve substantially the same quality of the image formed on the first side and the image formed on the second side.
However, such image processing imposes a heavy burden on a controller in the document reader and, hinders in shortening a time after image reading till image outputting.
In order to solve the above-described problems, it has been devised that image reading means of the optical reduction system for reading the image formed on the second side of the document, having performance similar to that of the image reading means of the optical reduction system for use in reading the image formed on the first side is replaced with the image sensor of the contact type.
As described above, the image reading means for reading the image formed on the second side is disposed on the way of the document conveyance path in the ADF repeatedly opened or closed with respect to the platen to generate vibration every time. Therefore, if the image reading means of the optical reduction system is disposed on the way of the document conveyance path in the ADF, there is induced a fear that an optical axis cannot be constantly kept caused by the deviation of an inclination angle of a mirror due to the repeated vibrations. It is indispensable to conduct a periodic maintenance in order to constantly keep the optical axis.
However, if the image reading means of the optical reduction system is merely disposed on the way of the document conveyance path in the ADF, there may occur an inconvenience that the maintenance of the image reading means of the optical reduction system cannot be conducted unless the ADF is disassembled every maintenance.
This not only requires much labor and skill for a maintenance service person but also forces a user to suffer an inconvenience of a long waiting time after the start of the maintenance till its completion.